Generally, the present invention relates to a fixed bed catalyst system for use in hydrotreating processes, and, in particular, a catalyst system comprising a physical mixture of catalyst particles with a large void and particles with a low void.
Hydrotreating of crude feedstocks encounters frequently problems caused by solid contaminants in the feedstock.
In many refineries, serious problems arise in hydrotreating reactors showing liquid maldistribution and pressure drop build-up due to plugging of the reactors by those solid contaminants.
If solid material of certain particle size is present in a feed-stock, it will deposit in the catalyst bed. An appropriate grading technology is necessary in order to control the deposition in such a way that the life of the catalyst charge is determined by lack of catalyst activity rather than flow restrictions or reactor plugging. Traditionally control of reactor contamination have been made by applying a grading of large particles on top of smaller particles. Both particle sizes used have almost the same void fraction, but the average size of the void spaces are different and the ability to retain deposits is thus different. By such a grading procedure, the deposits are allowed to penetrate deeper in the bed, and a higher contaminant uptake can, therefore, be attained before the catalyst bed is plugged off. The particle size procedure has the obvious draw back that the part of the reactor, where the accumulation occurs, has a limited capacity for contaminant storage. Exchange of large low void material with high void material, e.g. rings in the reactor top, have to a large extent improved the capacity for contaminant storage without having bed plugging. However, situations exists where the size of the contaminant particles is small and/or the feedstock contaminant level is high, and where a layer of large particles/high void material does not have adequate ability to retain contaminants and plugging of subsequent layers can thus occur unless very large volumes of large particles/high void material are used.